The present invention relates to a liquid crystal device, particularly a bistable liquid crystal device, having addressing means for providing a plurality of intermediate levels of light transmission. The invention also comprises a method of addressing a liquid crystal device and an arrangement for addressing a liquid crystal device.
In the field, particularly, of ferroelectric liquid crystal devices it is known to utilize a technique known as temporal dither in order to provide one or more intermediate states of light transmission between maximum transmission (generally referred to as white) and minimum transmission (generally referred to as black). By switching a pixel of the display to white for only a fraction of the frame, a grey level is obtained. The switching rate is sufficiently fast that the portions of time spent in the white state and the black state are perceived by the human eye to constitute a level of grey.
In a liquid crystal device array there are typically a first set of electrodes (or row electrodes) arranged on a first substrate of the device and a second set of electrodes (or column electrodes) arranged on an opposite substrate of the device. These sets of electrodes generally comprise electrodes arranged parallel to one another but at right angles to the electrodes in the other set. The intersection between a row electrode and a column electrode defines a picture element or pixel of the array. Each pixel of the array can be uniquely addressed by applying a scanning signal to each of the row electrodes in turn, while a data signal is applied to each of the column electrodes. The data and scanning signals must be carefully selected so that only those pixels in the row to which the scanning signal is applied will adopt the state as a consequence of the data signal. Once a scanning signal has been applied to all of the row electrodes the process can start again with potentially different data signals applied.
The scanning signal typically comprises a blanking pulse and a strobe pulse. The blanking pulse operates independently of the data signal to place all of the pixels in a particular row in a known state (typically the black state). Once the blanking pulse has altered the state of any pixels in the row which have previously occupied the other state, a strobe pulse is applied to the row electrode simultaneously with a data signal. The data signal may be selected from two or more possible data signals to place the pixel in the desired state.
In order to generate a grey level, then at least two strobe signals have to be applied to each row (for temporal dither). These signals are spaced within the frame time of the array device in order to permit different levels of grey to be obtained. FIG. 2(b) of the accompanying drawings shows the scanning signal applied to a row electrode in a prior art temporal dither arrangement. The frame is broken down into three durations of relative lengths 1:4:16. Before each of the divided time segments a blanking pulse Ba, Bb, Bc is applied to the row electrode to place all of the pixels in that row in a known state. Then a strobe pulse Sa, Sb, Sc is applied to the row electrode and, in combination with a data signal (not shown), can be used to place the pixel in either of the possible states. By applying the appropriate data signals to co-operate with the strobe pulses S any permutation of the grey levels in the ratios 1:4:16 may be provided. This prior art arrangement is used in conjunction with a 1:2 spatial dither arrangement (i.e. the pixel is physically sub-divided into one third and two thirds of its total area respectively) and this combination provides a total of 64 possible levels (including white and black). This will be discussed in more detail later.
A problem with this prior art arrangement is that it suffers from a lack of contrast and brightness. The reason for this is that the blanking pulses last for some time and, assuming that the display is blanked to black, result in a finite portion of the frame time during which the pixel is black rather than occupying the desired state. Consider the situation where a white (maximum light transmission) state is required. Each of the strobe pulses S will be coincident with a data signal which places the pixel in a white state, Therefore, the pixel will occupy the white state throughout as much of the frame time as is possible. However, for a short period of time prior to each strobe pulse S, the pixel will be in the dark state because of the presence of blanking pulses B. In addition, even when blanking black, transmission response causes a decrease in contrast due to small transmission during the application of the blanking pulse B.
It is an object of the present invention to provide a liquid crystal device, a method of addressing a liquid crystal device and an arrangement for addressing a liquid crystal device which ameliorates the above disadvantage.
According to a first aspect of the present invention, there is provided a method of addressing a liquid crystal device having a first plurality of electrodes and a second plurality of electrodes defining a plurality of pixels at the intersections between at least one of the first plurality of electrodes and at least one of the second plurality of electrodes, the method comprising applying one frame of a scanning signal to one of the first plurality of electrodes, applying a data signal to at least one of the second plurality of electrodes, one frame of the scanning signal comprising n strobe portions, where n is an integer greater than 1, for co-operation with the at least one data signal to address one of the plurality of pixels, and at least one blanking portion, the number of blanking portions not exceeding (nxe2x88x921).
The invention is based on the realization that the device can provide a useful number of grey levels without requiring a blanking pulse before each strobe pulse S.
According to a second aspect of the present invention, there is provided a liquid crystal device having a first plurality of electrodes and a second plurality of electrodes defining a plurality of pixels at the intersections between at least one of the first plurality of electrodes and at least one of the second plurality of electrodes, further comprising means for applying one frame of a scanning signal to one of the first plurality of electrodes, means for applying a data signal to at least one of the second plurality of electrodes, wherein one frame of the scanning signal comprises n strobe portions, where n is an integer greater than 1 , for co-operation with the at least one data signal to address one of the plurality of pixels, and at least one blanking portion, the number of blanking portions not exceeding (nxe2x88x921).
According to a third aspect of the present invention, there is provided an addressing arrangement for a liquid crystal device having a first plurality of electrodes and a second plurality of electrodes defining a plurality of pixels at the intersections between at least one of the first plurality of electrodes and at least one of the second plurality of electrodes, the arrangement comprising means for applying one frame of a scanning signal to one of the first plurality of electrodes, means for applying a data signal to at least one of the second plurality of electrodes, wherein one frame of each of the scanning signal comprises n strobe portions, where n is an integer greater than 1, for cooperation with the at least one data signal to address one of the plurality of pixels, and at least one blanking portion, the number of the blanking portions not exceeding (nxe2x88x921).
At least two of the strobe portions of the scanning signals may have different polarities.
Preferably, first and second scanning signals are applied to adjacent ones of the first plurality of electrodes (row electrodes). This increases the number of possible grey levels for a given number of strobe portions per frame. The scanning signals may blank their respective rows to the same state or to opposite states.
Simultaneous strobe portions of the first and second scanning signals may have opposite polarity.
Preferably, three data signal types are provided, the third providing the same response regardless of the polarity of the strobe signal with which it is applied.
Further features of the invention are set out in the accompanying dependent claims and will be apparent to the skilled person from the detailed description which follows.